Knapsack cooler



FIG. 1 is a perspective view of a knapsack cooler showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a bottom plan thereof; and

FIG. 6 is a rear elevational view thereof. 

The ornamental design for a knapsack cooler, as shown. 